More Dark Stars than Stars in Milky Way

For some time, astrophysicists have argued over how many Dark Stars there might be in the galaxy, with varying opinions. (Note that astronomers use several different names for these objects: sub-brown dwarfs, Y Dwarfs, ‘planemos’). In this short article, I argue that new evidence presented about the stellar populations of open star clusters point towards there being more Dark Stars than stars in our galaxy.

When I use the term ‘Dark Star’ in my book (1) and internet articles, I’m generally referring to gas giant planets/ultra-cool dwarf stars which are several times more massive than Jupiter, up to perhaps ~13 times as massive (at this point, the gas giant begins to burn deuterium and is reclassified as a brown dwarf). Most examples of these objects (perhaps more than a few million years old) are essentially dark. By contrast, very young examples light up more brightly, because they still retain some heat from their formation. It’s a curious quirk of nature that these sub-brown dwarfs are actually smaller in size than Jupiter, despite being heavier. Because these objects are so small, and so dim, they are extraordinarily difficult to observe. Some have been found, but they are usually either extremely young (and therefore still burning brightly), or are exoplanets discovered orbiting parent stars (and so detectable through gravitational ‘wobble’ effects, or other means of finding massive exoplanets).

It has been my contention for some time that the populations of these objects are significantly underestimated. It is recognised generally that these ultra-cool dwarf stars may be free-floating objects in inter-stellar space, often as a result of having been ejected from young star systems as the fledgling planets in those systems jostle for position. Opinions about their numbers vary greatly among astrophysicists. There may be twice as many of these objects as stars, according to studies involving gravitational microlensing surveys of the galactic bulge (2). Other studies conflict with this conclusion, arguing that there may be as few as 1 object of 5-15 MJup size per 20-50 stars in a cluster (3). This discrepancy is important because the difference is perhaps as high as two orders of magnitude, and this ultimately affects our understanding of how many free-floating Dark Stars we can expect to find out there.

Their mass, lying between that of Jupiter and the deuterium-burning limit at about 13 MJup (4) seems to single Dark Stars out as rather special objects:

“An abrupt change in the mass function at about a Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.” (2)

Therefore, if the number of free-floating sub-brown dwarfs (also sometimes known as “planemos”) is on the high end of expectation, then it means that there are also likely to be far more of these objects in wide, distant orbits around their parent stars. This, in turn, increases the likelihood of there being a similar Dark Star object (or more) in our own immediate solar neighbourhood.

These free-floating sub-stellar Dark Stars are extremely difficult to locate, certainly in visible light. Within our own galactic neighbourhood, astronomers stand a better chances of imaging them using near infra-red sky searches, like WISE and 2MASS, as the sub-brown dwarfs’ intrinsic size and density should produce sufficient heat to allow them to stand out from the frigid background of space. Even so, their age (1-10 billion years old) makes then exceedingly faint. It is perhaps no surprise, then, that the trawl across the sky by WISE for these local objects was not particularly successful (5).

But these objects need not be distributed randomly through interstellar space. Studies of open clusters and associations of young stars have received much attention in recent years. These clusters may well host the brightest examples of low-mass sub-stellar objects in the solar neighbourhood. The youngest of these associations within 100 parsecs is TW Hya – more usually known as ‘TWA’. This star cluster, located about 100 light years away, contains a few dozen 10-million-year-old stars, all moving together through space (6). It is known that many of TWA’s low-mass objects are still missing – despite careful study by astronomers for over a decade. They were too faint to have been detectable by ESA’s Hipparcos mission, which accurately determined the astrometry of a huge number of stars during the lifetime of its mission.

A new census of TWA has included computer simulations to try to fill in some of the gaps in the sub-stellar populations of this open cluster of stars (a bit like working out how many winnows there might be lurking within a large school of fish). The scientific team seemed particularly interested in working out whether the cluster might contain additional hidden objects in the ∼5–7 MJup range – very much in the ‘Dark Star’ territory! This interest has been prompted by recent discoveries by the near-infrared Two-Micron All Sky Survey (2MASS) of two such objects within the moving, open cluster:

“Two recent discoveries in particular demonstrate the interest of TWA as a laboratory for understanding this isolated planetary-mass population,”said Carnegie’s astronomer Jonathan Gagné and co-authors. “2MASS J11193254-1137466 and 2MASS J11472421-2040204 are both candidate members of TWA with spectral types L7 that display signs of youth, and with estimated masses as low as 5-7 Jupiter masses. Their close distances to the Sun place them at the nearer side of the TWA spatial distribution.”

“In order to determine whether or not there are more stand-alone planetary mass-sized objects like these in TWA, the astronomers undertook the calculation of an astronomical measurement called the initial mass function [IMF]. This function can be used to determine the distribution of mass in the group and to predict the number of undiscovered objects that might exist inside of it.” (7)

The result of this calculation is that there are likely to be an additional 10 objects in the ∼5–7 MJup range in the TWA cluster (and perhaps more than 20, at the top end of their theoretical estimate). This large number of potential objects has surprised the astrophysicists, as it surpasses the numbers of low-mass objects which might be expected given the populations of stars in the census of TWA. But the new estimate is in keeping with other studies of galactic interstellar populations of sub-stellar objects of this size:

“This is much higher than what would be expected based on a log-normal IMF that is anchored on the higher-mass population of TWA. This possible over-density of objects in the planetary-mass regime is surprising, but consistent with recent estimates for the space density of objects at the deuterium-burning limit… (8), the recent discovery of a cold, planetary-mass Y dwarf at only 2 pc from the Sun (9), as well as results from micro-lensing surveys (1).” (10)

So, it looks like the number of free-floating Dark Stars may well be more than the number of actual stars in our galaxy after all.